Python unittests: how to get an AsyncMock to block until a certain point

I have some async Python code I want to test, which include something like

async def task(self):    while True:        await self.thing.change()        ...        self.run_callbacks()

In my test file, I'm starting the task in the setUp() function like so:

def setUp(self):   ...   self.loop = asyncio.get_event_loop()   self.task = self.loop.create_task(self.object_under_test.task())

then wanting to verify that the callbacks ran:

def test_callback(self):    callback = mock.MagicMock()    self.object_under_test.add_callback('cb1', callback)    # ???    callback.assert_called_once()

Where I'm stuck is how to mock thing.change() in the task. It needs to block until I tell it to (at ???) at which point it will return. If I mock it after starting the task, then it won't make any difference as the task is already waiting for the un-mocked function to return. If I mock it prior to starting the task, then it will always return something and not block.

Any suggestions how I might achieve this?